Some middlemen request or prefer tips, but middlemen who insist on money tend to find themselves rapidly out of work due to the number of trustworthy middles who offer their services for free. (: It's a matter of personal preference, really, but you probably won't run into anyone requiring a fee, and if you do, you should find someone else. (:

As for tips, generally whatever you can afford is fine. Since so few people do it, receiving a tip at all is nice in itself. I myself have received tips as low as 100 credits, and as high as about half a million.